Two types of bicycle transmissions are standard. When a large-number of ratios is needed, both the pedal shaft and back wheel are provided with a plurality of coaxial sprockets of different sizes. A chain spans the two and a derailleur mechanism is provided for shifting the chain on both the front and back sprocket. For hill climbing a small-diameter sprocket on the pedal shaft is connected to a large-diameter sprocket on the rear wheel, and for downhill or on-the-level travel the relationship is changed or reversed. Bicycles having fewer ratios, typically a maximum of three or four, use a planetary-gear transmission inside the rear driven-wheel hub. This latter transmission can be made to operate automatically, in response to torque or force. Both these systems have the disadvantage that only a limited number of ratios is available and the changeover from ratio to ratio involves a step.
Accordingly it has been proposed to replace the chain with a belt and use a belt drive with a steplessly variable input/output ratio having a pair of wheels or pulleys of variable effective diameter or pitch coupled together by the belt. Each wheel is formed of at least three radially displaceable segments that are urged radially outward by springs. The spring forces are such that as tension in the belt increases, that is when torque applied to the drive wheel or pulley increases, the drive-wheel diameter decreases in diameter and the rear driven-wheel diameter increases, and vice versa.
As described in German patent 93,896 issued 3 Oct. 1896 to E. A. Ashcroft such a steplessly variable belt drive has spiral springs in the drive and driven wheels, one spiraling in one direction relative to the respective axis and the other spiraling oppositely. The inner ends of the springs are secured to the respective hubs and the outer turn of each spring bears on the radially inner faces of the respective segments. Each wheel has guide plates on which the radially displaceable segments are supported. This structure is cumbersome and difficult to accommodate on a bicycle where space and weight are at a premium, and the rubbing of the segments on the guide plates causes the system to respond sluggishly and malfunction when dirty.
German patent 122,517 issued 13 Apr. 1900 to L. Rottenberg has a spring arrangement involving levers and springs for spreading the pulley segments, and French patent 1,188,899 issued 16 Mar. 1959 use elastically deformable segments for the same effect. Similarly, German patent 642,664 issued 5 Oct. 1935 to K. Heidsieck has pivotal arms whose outer ends define the effective diameter of the pulley and whose positions are controlled by a lever system in turn operated by a planetary-type adjustment mechanism, with no automatic adjustment.
All these systems are relatively bulky and complex. None of them can be retrofitted on an existing bicycle to replace the standard derailleur system and, in effect, provide the bicycle with an automatic transmission.